1. Field of the Invention
This invention relates to an optical scanning device wherein two plane correction mirrors are used to optically correct scanning line distortion caused by misalignment of the deflecting reflecting surface of a mechanical type optical deflector such as a multifaceted rotating mirror.
2. Description of the Prior Art
Optical scanning devices which employ mechanical optical deflectors such as multifaceted rotating mirrors, galvanometer mirrors, or bimorph mirrors are widely used. With such optical deflectors, the deflecting reflecting surface is rotated or swivelled about its axis, but there have been problems such as that the precision with which the deflecting reflector is fixed with reference to the axis, and the precision of fabrication of the surface of the deflecting reflector, has been poor, causing misalignment of the deflecting reflecting surface, deviation of the axis itself, and the like, causing distortion of the scanning lines. When an optical scanning apparatus is being used for recording or read-out of image information, such distortion of the scanning lines greatly reduces the precision of the image information recording or read-out.
There has been proposed a number of methods for eliminating this scanning line distortion. In one such method a correctional optical deflector such as an acousto-optical modulator is interposed in the light beam used for scanning (Japanese Laid-open Patent Application Nos. 47-33642, 53-146643, and 53-111745). However, a problem with this method has been the complexity of the system for controlling the correctional optical deflector.
Another proposal has been to use a pair of anamorphic optical systems whereby the scanning light beam is reflected by the deflecting reflector as a line image parallel to the direction of rotation of the deflecting reflector and scans again as a light spot (Japanese Patent Publication No. 28666/1977). However, a problem with this has been that the optics is complex and adjustment is difficult.
In another proposal (U.S. Pat. No. 3897132), two plane correction mirrors which intersect each other are arranged so that the line of intersection is at right-angles to the axis of rotation of the deflecting reflecting surface, and the light beam reflected by the deflecting reflector is returned to the deflecting reflecting surface by these plane mirrors, so as to have it reflected twice by the deflecting reflector. However, with this method, if the incident light beam and the scanning light beam going to the scanning surface are not both at right-angles to the axis of rotation of the deflecting reflecting surface, the scanning beam distorts into a curve. When the incident light beam is not at right-angles to the axis of rotation of the deflecting reflecting surface, the first reflection from the deflecting reflecting surface produces a curvature in the reflected light beam, and curvature is again produced in the light beam when it is returned to the deflecting reflecting surface by the pair of plane correction mirrors and reflected a second time, so the end result is that the scanning lines retain a large curvature.